This invention relates to proximity printing, and more particularly, to the photolithographic printing of mask patterns representative of electronic circuit elements on photosensitized semiconductor wafers.
The fabrication of semiconductor integrated circuits requires the repeated projection of light through different masks onto a semiconductor wafer coated with a photosensitive film. After each exposure and appropriate development and etching, the film itself then constitutes a mask for selective processing of the wafer such as diffusion, metallization or etching.
The three principal techniques for such photolithographic printing are contact printing, in which the mask is in physical contact with the film; projection printing, in which an image of the mask is projected onto the film; and proximity printing, in which a small gap is maintained between the mask and the wafer as is desirable for avoiding damage to mask and thereby extending mask lifetime. Good resolution resulting in accurate exposure of the fine details used in modern photolithographic masks requires that this gap be maintained within stringent uniformity tolerances. This gap is presently established by any of various forms of fixed spacers. Such spacers almost invariably give a slightly nonuniform support of the mask over the wafer, resulting in slight nonuniformities in the separation gap. Also, inherent deviations from flatness by the semiconductor wafer and mask surfaces are a prime source of gap nonuniformities.